Can the author of software make it proprietary after making it gpl?
Yes, the author (or whoever holds the copyright) can do as they please, changing the license at will. But only close up later versions, what was distributed under an open source license stays forever open.
Case in point is SSH: It was open source up to version 1, version 2 (clearly a development on version 1) is closed. OpenSSH took version 1 (still open source) and created an extension handling the new protocol, released as open source.